There are many great hackers in the world. Some have written code that turned out to be critical to our everyday hacking lives. Zed seems to think that because he wrote one piece of software that has been used a lot (its really just a small blip on the "used a lot" scale) that he has the right to constantly remind everyone of this. Seriously, do the authors of VIM, openssh, firebug, etc... constantly remind us that we owe a debt to them by listening to their rants?
The bottom line is if Zed had not have written mongrel, someone else would. No hacker is irreplaceable.
I don't know if the Merb/Rails merger would have happened if the ego-inflating hype around Rails was still on. In that sense, maybe Mongrel's main benefit (IMHO) was that it gave Zed credibility which made his "Rail Ghetto" rant significant.
I agree everyone is replaceable, and yet - surprisingly small number of people out of those who have the skills to build such projects actually bother to do it. It's easy to know how to build something, hard to actually do it, from start to finish.